This invention relates to an electrical system and, in particular, to an electrical system including circuitry for suppressing small, noise-like, input signals.
A problem which exists in the art and which is solved by the present invention may be best explained with reference to FIG. 1 which shows a system designed to sense the "knock" of an engine and means for generating signals to correct for the knock. A knock sensor 10, which may generally be a microphone and which is located on or near an automobile engine, produces a knock signal (ek) in response to the "knocking" of the engine. The signal (ek) is then applied to an amplifier 12 whose output is applied to the input of an anti-aliasing filter 13 whose output is applied to the input of a programmable gain stage 14. In response to the knock signal ek the programmable gain stage 14 produces at its output an in-phase signal (ek1) and an out-of-phase signal (ek1b). The signal ek1 is applied to a bandpass filter 16a and the signal ek1b is applied to a bandpass filter 16b. The output of filter 16a produces a signal identified as V.sub.IN and the output of filter 16b produces a signal identified as V.sub.INB. Theoretically, in-phase signal V.sub.IN should be the exact complement (or inverse) of the out-of-phase signal V.sub.INB. Input signals V.sub.IN and V.sub.INB are then applied to a rectifier section 18 which controls the application of the signals V.sub.IN and V.sub.INB to an integrator 20. Integrator 20 includes a positive signal integrator 20a and a negative signal integrator 20b. The rectifying circuit 18 includes circuitry for comparing V.sub.IN and V.sub.INB and switches for enabling the positive going portion of signals V.sub.IN and V.sub.INB to be applied to integrator 20a and the negative going portion of signals V.sub.IN and V.sub.INB to be applied to integrator 20b. As a result, the outputs of integrators 20a and 20b function to increase the positive and negative amplitude of the knock signal over selected integrating intervals.
The outputs of integrators 20a and 20b are fed to a differential to single-ended amplifier whose output charges a storage capacitor C24 whose potential is used to drive a buffer 26 whose output is then used to control (reduce) the engine knock.
A problem exists when there is no knock signal (e.g., the knock sensor is disconnected) or when the knock signal is very small. It is virtually impossible to manufacture a programmable gain stage (e.g., 14) such that the circuit sections producing the in-phase and out-of-phase signals and their corresponding filters (e.g., 16a, 16b) are totally identical. No matter how carefully the circuit and its components are made, there are differences (albeit small) in the components and their lay out. Consequently, when the knock signal (ek) is very small or non-existent, a net difference or offset will nevertheless exist between V.sub.IN and V.sub.INB. Although the difference and offset may be small, a significant error may result. This is because the outputs of the bandpass filters (16a, 16b) are fed to a rectifier section 18 to control the application of V.sub.IN and V.sub.INB to integrators 20a and 20b. The difference between V.sub.IN and V.sub.INB due to noise and offsets produces a "false" signal which increases in time, magnifying the effect of the false signal. Therefore, it is an object of this invention to override and eliminate the effect of "low level" and "offset" signals.